My U.S. Pat. No. 4,917,708, issued Apr. 17, 1990, discloses an indirect calorimeter, or oxygen consumption meter, which may be used to measure the resting energy expenditure of a subject. This measurement is important for determination of the proper caloric content for feedings of hospitalized patients and also is useful in connection with weight loss diets since the basal energy requirement may vary during the period of the diet. Similarly, knowledge of caloric expenditure and oxygen consumption during exercise are useful for cardiac rehabilitation and athletic training.
That patent discloses a calorimeter which utilizes a unidirectional flow meter operative to generate electrical signals proportional to the respiratory gases passing through it, a carbon dioxide scrubber operative to remove CO.sub.2 from the exhaled gas and valving and conduits connecting the flow meter and the scrubber between a source of respiratory gases, which may be either the ambient air or some form of positive pressure ventilator, and a patient mouthpiece. The inhaled air has a negligible content of carbon dioxide and the exhaled gas contains lung-contributed carbon dioxide of essentially the same volume as the oxygen consumed by the subject. Accordingly, the difference in volumes between the inhaled and scrubbed exhaled gases passed through the flow meter provides an indication of patient's oxygen consumption. By integrating these differences over a test period, which may last for several minutes, an accurate measurement of the subject's oxygen consumption during the trial may be obtained.
My U.S. patent application Ser. No. 726,922, filed Jul. 8, 1991, discloses a calorimeter utilizing a bidirectional flow meter which passes the inhaled gases before they are scrubbed for CO.sub.2 and the exhaled gases after they are scrubbed, resulting in a simplified design and the potential for disposability after a single use, eliminating the requirement for sterilization.
One embodiment of that invention employs a capnometer disposed in the flow path between the subject's mouthpiece and the CO.sub.2 scrubber so that the exhaled gases are passed through the capnometer before being scrubbed. The capnometer generates an electrical signal which is a function of the CO.sub.2 concentration of the exhaled gases. The electrical output of the capnometer, along with the flow meter signal, may be used to generate the ratio of carbon dioxide to consumed oxygen, or the Respiratory Quotient (RQ) as well as the Resting Energy Expenditure (REE), another important measure of a subject's metabolism.